Buffering a hierarchical index of multi-dimensional data

ABSTRACT

Methods are provided for buffering nodes of a hierarchical index (e.g., R-tree, bang file, hB-tree) during operations on multi-dimensional data represented by the index. The methods are particularly suited for query operations, and a different method may be more suitable for one pattern of queries than another. Where queries are distributed in a relatively uniform manner across the domain or dataspace of an index, a node-area buffering method is provided. In this method nodes are cached or buffered in order of their respective areas (e.g., their minimum bounding areas), and a node having a smaller area will be replaced in cache before a node having a larger area. When, however, queries are not uniformly distributed, then a least frequently accessed buffering technique may be applied. According to this method statistics are maintained concerning the frequency with which individual index nodes are accessed. Those accessed less frequently are replaced in cache before those accessed more frequently. Yet another, generic, buffering strategy is provided that is suitable for all patterns of query distribution. In accordance with this method, whenever a node must be removed from cache in order to make room for a newly accessed node, cached nodes are compared to each other to determine which provides the least caching benefit and may therefore be ejected. A comparison may involve three factors—the difference in the nodes&#39; areas, the difference in the frequency with which they have been accessed and the difference between their latest access times. These factors may be weighted to give them more or less effect in relation to each other.

RELATED APPLICATION

This application is a continuation-in-part of pending U.S. application Ser. No. 09/322,901, filed May 29, 1999.

BACKGROUND

This invention relates to the field of database management systems. More particularly, a system and methods are provided for indexing multi-dimensional data, storing such data in a relational database management system and efficiently retrieving the data upon demand.

Various methods of managing collections of data (e.g., databases) have been developed since data was first stored in electronic form. From initial systems and applications that simply collected data in one or more flat database files to present sophisticated database management systems (DBMS), different solutions have been developed to meet different requirements. Early solutions may have had the advantage of simplicity but became obsolete for a variety of factors, such as the need to store large—even vast—quantities of data, a desire for more sophisticated search and/or retrieval techniques (e.g., based on relationships between data), the need to store different types of data (e.g., audio, visual), etc.

A database may be considered distinct from a DBMS, in that a DBMS, as the name implies, includes utilities for accessing, updating and otherwise managing or operating a database. As the amount and complexity of data stored in databases has increased, DBMS design and development efforts have increasingly focused upon the ability to organize, store and access data quickly and efficiently. As a result, today's database management systems can be very effective in managing collections of linear information such as inventory, customer lists, etc.

With such linear (or uni-dimensional) data—data that varies in value in a single dimension—determining and maintaining relationships between data elements is relatively easy. For example, one value or data point (e.g., a price, a quantity) can easily be compared to another to determine which is “greater” or which ones match a particular query. The ordinal nature of linear data therefore readily lends itself to basic indexing and subsequent storage, search, retrieval and other data management operations. In particular, the appropriate point for insertion, retrieval or deletion of a data element in a database of linear data may be found with great facility by referring to a table or other data index.

In short, today's database management systems have been designed to manage linear data very effectively. Present database management schemes are still poorly suited, however, for managing data that are multi-dimensional in nature. Geographic data, for example, may be meaningful only when expressed in at least two dimensions (e.g., latitude and longitude) and can thus be considered “inherently” multi-dimensional Because such data can vary in value in more than one dimension, the relationship between selected geographic points is more complex and, unless a particular reference dimension or other criteria is chosen, one point cannot automatically be considered “greater” or “less” than another. The difficulty in expressing relations among inherently multi-dimensional data makes indexing such data (e.g., for storage and retrieval) more complicated and can also complicate schemes for buffering the data as queries are processed.

Closely related to inherently multi-dimensional data are multi-dimensional data that may also be termed “multi-attribute” in nature. Multi-attribute data may be defined as information that possesses multiple defining characteristics that are not inherently related. For example, sales data may be considered multi-attribute in nature if characteristics such as time of sale, region of sale, salesperson, product sold, type/model of product, and so on are recorded for each sale. These data become multi-dimensional or multi-attribute in nature when queries are made or reports are desired that specify range predicates in two or more of the data attributes (e.g., the sales made by Salesperson A during the previous month, the best time of year for selling a particular product). Today's database management systems are, unfortunately, not designed to organize this data in a manner that enhances the ability to retrieve those data items satisfying a particular multi-dimensional query pattern.

Present techniques for dealing with (e.g., indexing, storing, retrieving) multi-dimensional data often involve attempts to translate the data into a single dimension so that existing systems may be used. These techniques may fail to maintain important spatial relationships, however, thus adversely affecting the ability to respond rapidly to multi-dimensional queries. For example, linear quadtrees and Hilbert R-trees transform multi-dimensional data into a single dimension and then construct B-trees on the linearized data. Although these schemes may be adequate for two-dimensional data, linearizing data having three or more dimensions may result in an unacceptable loss of a spatial or other data relationship.

Meanwhile, the number and types of applications that use multi-dimensional and multi-attribute data—such as geographic information systems (GIS) and computer-aided design and manufacturing (CAD/CAM) systems—continue to grow. A GIS application may work with maps or charts in which spatial data are expressed in two or three dimensions (e.g., latitude, longitude and, possibly, altitude). Similarly, in CAD/CAM applications, products such as printed circuit boards for computer systems may be designed using rectangular areas or cubical volumes. A person using one of these applications may select an area of interest that contains or intersects one or more elements. The application must be able to accurately identify those elements and facilitate modification of the data.

Multi-media applications, in which audio and visual elements are combined in one database, are another area that can benefit from efficient storage of multi-dimensional data. For example, an element of a graphical image may require multi-dimensional data for accurate representation. In particular, graphic images may be described in terms of numerous spectral characteristics (all or some of which may be inherently inter-related). Thus, in a graphical element that embodies a combination of colors (e.g., some mixture of red, green and blue pixels on a computer display) the different colors of the element may be represented in different dimensions. Each dimension's value could, for example, represent the relative proportion of the corresponding color within the element or the number of pixels within the element that have that color. Accurate representation of this data would allow an application to easily identify elements that have similar coloring.

Because effective methods of organizing multi-dimensional data in a DBMS have been generally unknown, applications that use such data have been unable to reap the advantages offered by today's database management systems, especially relational database management systems (RDMBS). Among those advantages are backup and recovery utilities, robust security, and controls allowing concurrent data access. Developers of these applications have, instead, had to rely upon other methods of indexing and storing such data.

What is needed then is a method of organizing multi-dimensional/multi-attribute data in a DBMS, particularly a relational DBMS, in order to reap the advantages of sophisticated management controls (e.g., concurrent access to the data) without sacrificing spatial relationships. Advantageously, such a DBMS would provide for efficient organization of the data to facilitate rapid retrieval.

Further, an efficient method or system for facilitating retrieval of multi-dimensional/multi-attribute data is also needed. In particular, a suitable technique is required for buffering data as database queries are processed.

SUMMARY

In addition to providing for effective management of data that are inherently multi-dimensional (e.g., geographic, multi-media), an embodiment of the invention also provides for the storage and management of linear data having multiple attributes. For example, a database of sales figures may be indexed according to attributes such as product, time, salesperson, geographic region, etc.

In addition to providing for effective management of data that is inherently multi-dimensional (e.g., geographic, multi-media), an embodiment of the invention also provides for the storage and management of linear data having multiple attributes. For example, a database of sales figures may be indexed according to attributes such as product, time, salesperson, geographic region, etc.

In one embodiment, a set of multi-dimensional/multi-attribute data items is indexed by recursively clustering the data items into smaller collections until each cluster can be stored (i.e., indexed) in a single leaf node of a hierarchical (e.g., tree-structured) index. In particular, when the set of data items—or a subset thereof—is too large to fit in a single leaf node, a suitable dimension/attribute by which to divide the data items is selected and the set or subset is divided accordingly. The capacity of a node may be specified as a fanout characteristic of the index or may be determined by a parameter of a suitable physical storage device (e.g., the capacity of a disk page).

The selected dimension or attribute in which to divide a data set or subset may be the one having data item values exhibiting the greatest variance or range. Alternatively, a dimension may be selected based upon an expected or specified query pattern. Illustratively, when a dividing dimension is selected, the data items are sorted in that dimension and then divided into two or more subsets that contain equal or nearly equal numbers of data items. After leaf nodes are constructed for clusters of data items, intermediate nodes and, finally, a root node may be constructed to complete the index. Each higher-level node is designed to encompass or contain its children nodes.

A hierarchical index (e.g., an R-tree index) constructed to contain multi-dimensional or multi-attribute data may be stored in a database environment—such as a relational database management system. In one embodiment of the invention a first object or table in a database is configured to store information concerning the index (e.g., its dimensionality, fanout) and possibly an identifier (e.g., an address or storage location, a unique node identity) of a root node of the index. A second object or table is configured to store a record or row for each node of the index. The multi-dimensional data items may be stored in one or more objects or tables, in the same or a different database.

In the second object or table mentioned above, each record for an index node may consist of items such as: a unique identifier of the corresponding node, an identifier of a parent node, an identifier of a sibling node, a measure of the number of children of the node, and an entry for each child. In one embodiment of the invention each entry for a child node includes an identifier of the child, which may be a data item (if the node is a leaf node) or another node. Illustratively, each index record also includes a bounding region or area that encompasses the data item (if the node is a leaf node) or all data items that descend from the node (i.e., all data items below the node that are connected to the node through one or more intervening nodes).

In an embodiment of the invention in which a user's query will likely match one of a set of known query patterns (possibly within a range of accuracy), the data items may be clustered for indexing in an appropriate and corresponding manner. Thus, if a possible query pattern expresses a particular order or hierarchy between the dimensions/attributes of the data, the data items may be divided and clustered accordingly in order to create an index tailored to facilitating an efficient response to an actual query. Multiple indexes may thus be created (and stored in a database) for a given set of data items, with each index reflecting a different scheme of clustering.

In one or more embodiments of the invention techniques are provided for buffering index nodes during operations (e.g., additions or deletions of a data item, queries, reorganizations) on multi-dimensional and/or linear data. A first buffering technique buffers nodes in the order of the size of their bounding regions or areas. A node's bounding region may be stored, with an identifier of the node, in an entry in its parent node. A root node's bounding region is considered to encompass the entire set of multi-dimensional data items indexed within the hierarchical structure. This technique is particularly well suited for use when the data operations are distributed relatively uniformly across the dataspace.

A second buffering technique involves the collection of statistics concerning the frequency and/or recency with which nodes and/or data items are accessed (e.g., during queries and updates). Nodes may then be buffered based on the frequency or recency with which they have been accessed or the frequency or recency with which data items within their bounding regions have been accessed. The nodes that are buffered may change over time as additional operations are conducted. This second technique is well suited to environments in which data operations are not uniformly distributed within the dataspace.

In one embodiment of the invention index nodes are first buffered according to the size of their bounding regions. Then, after a meaningful number of operations have been performed, nodes are buffered according to the frequency or recency of access.

A third, generic, technique of buffering uses a combination of two or more of the node region, recency of access and frequency of access criteria to determine which buffered node is to be ejected from a buffer in favor of a newly accessed node. In one embodiment of the invention implementing this technique, all three of these criteria are applied. In this embodiment, two buffered nodes are compared at a time to determine which provides less caching or buffering benefit in accordance with the selected criteria. By successively comparing the losing node of one comparison with another buffered node, eventually the node providing the least caching benefit is identified.

In this embodiment, the node region criteria are measured by determining the difference between the two nodes' bounding regions; this difference may be divided by one of the two bounding regions in order to normalize this factor. The recency of access criteria is measured by calculating the difference in time between the last accesses of the two nodes; this difference may be normalized by dividing it by the size of the buffer or cache. The frequency of access criteria is measured by determining the difference between the number of times each node was accessed during a set of data operations. The values resulting from the three criteria are then accumulated, possibly after multiplying one or more of them by weighting factors. If the result is greater than a certain threshold (e.g., zero), then one node is considered to have less caching benefit and may therefore be ejected; otherwise, the other node will be ejected.

DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B is a block diagram depicting a set of multi-dimensional data items and one R-tree index that may be constructed to index the data items in accordance with an embodiment of the present invention.

FIG. 2 is a flowchart illustrating one method of storing a hierarchical index of multi-dimensional data in a relational database in accordance with an embodiment of the invention.

FIG. 3 depicts the division of a set of multi-dimensional data items into clusters, for storage in R-tree leaf nodes, in accordance with an embodiment of the present invention.

FIG. 4 is one R-tree index that may be constructed from the set of multi-dimensional data items depicted in FIG. 3 in accordance with an embodiment of the present invention.

FIG. 5 is a flowchart illustrating one method of indexing a set of multi-dimensional data in accordance with an embodiment of the invention.

FIG. 6A depicts the addition of a data item to a set of multi-dimensional data items in accordance with an embodiment of the present invention.

FIG. 6B depicts an R-tree index resulting from the expanded set of multi-dimensional data items of FIG. 6A in accordance with an embodiment of the present invention.

FIG. 7 depicts a nearest neighbor query in a set of multi-dimensional data items in accordance with an embodiment of the present invention.

FIG. 8A is a flowchart depicting methods of buffering a hierarchical index in a database environment having a uniform or non-uniform pattern of database operations in accordance with an embodiment of the invention.

FIG. 8B is a flowchart depicting a generic method of buffering a hierarchical index in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of particular applications of the invention and their requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

The program environment in which a present embodiment of the invention is executed illustratively incorporates a general-purpose computer or a special purpose device such as a hand-held computer. Details of such devices (e.g., processor, memory, data storage and display) are well known and are omitted for the sake of clarity.

It should also be understood that the techniques of the present invention might be implemented using a variety of technologies. For example, the methods described herein may be implemented in software executing on a computer system, or implemented in hardware utilizing either a combination of microprocessors or other specially designed application specific integrated circuits, programmable logic devices, or various combinations thereof. In particular, the methods described herein may be implemented by a series of computer-executable instructions residing on a storage medium such as a carrier wave, disk drive, or computer-readable medium. Exemplary forms of carrier waves may take the form of electrical, electromagnetic or optical signals conveying digital data streams along a local network or a publicly accessible network such as the Internet.

Introduction

A method and apparatus are provided for organizing multi-dimensional data in a relational or object-relational database management system, such as Oracle® Server by Oracle Corporation. In particular, methods and apparatus are provided for indexing multi-dimensional data items in a hierarchical index, storing the index in a database, and performing various operations on the index and/or data items. Further, methods are described for buffering contents of a hierarchical index during database operations such as queries to facilitate efficient retrieval of and access to data items and index nodes.

Embodiments of the invention described herein may be used to efficiently store, organize, manipulate and retrieve data for applications in the areas of geographical information systems (GIS), computer-aided design and computer-aided manufacturing (CAD/CAM), data warehousing, multi-media, etc. Various types of multi-dimensional data, such as geometrical, geographical, rectangular (e.g., elements of a CAD/CAM project), and geoimage data, plus data representing image, audio and video feature vectors and data possessing multiple attributes, may thus be stored or indexed in a database in one or more embodiments of the invention. In particular, the data items that are indexed and manipulated in an embodiment of the invention may be point data or non-point data (e.g., spatial in nature).

An embodiment of the invention may be installed as an integral part of any database server, whether part of a centralized computing environment, client-server environment or otherwise. The index and/or data may, for example, reside on a client in order to minimize the expense of network activity and access to storage devices. The processing and input/output burdens caused by single/multiple operations on the index may be shared using single/concurrent threads running on single or parallel processor machines operating with single/multiple storage devices.

For purposes of embodiments of the invention described herein, multi-dimensional data may possess any number of dimensions (e.g., two or greater). Multi-dimensional data may also include data that possess multiple attributes or values in multiple dimensions that are not inherently related. For example, geographic data is often expressed in terms of latitude and longitude (or some other coordinate system). Geographic data may thus be considered inherently multi-dimensional because a latitude or longitude value, by itself, cannot adequately identify a geographic feature.

Embodiments of the invention described herein are, however, also effective with data that are not inherently multi-dimensional but which may be accessed or manipulated on the basis of multiple attributes or values. For example, sales data may have attributes such as time (e.g., time of sale), product (e.g., item sold) and area (e.g., region in which a sale occurred). Although sales information may be retrieved using just one of these attributes, reports may be desired that specify range predicates on two or more of these attributes (e.g., all sales of a particular product in a particular region during a specific time period). Although a basic sales data item may not be inherently multi-dimensional, storing or indexing it so as to facilitate its multi-dimensional manipulation may allow more efficient use of the data. Thus, the term multi-dimensional may be used herein to refer to multi-dimensional and/or multi-attribute data.

Within a particular data dimension or attribute, a hierarchy of selectivities or granularities may be specified. In various embodiments of the invention, one or more indexes may be constructed and stored in a database for a set of data items by choosing different selectivities for one or more dimensions or attributes. For example, a region dimension of sales data may include values for both city and state. A first index may then be constructed on the basis of city-level selectivity in the region dimension and, for example, a year selectivity in a time dimension. Another index may be constructed using state and year selectivities.

R-Trees

One embodiment of the invention provides a method and apparatus for efficiently storing and maintaining an R-tree index in a database management system (DBMS). An R-tree index stored within a database in this embodiment supports virtually all update operations (e.g., addition or deletion of a data item, database reorganization) and query operations, plus other database operations such as backup, recovery, security, etc.

R-trees are often used to index data for applications such as those listed above because the R-tree structure has been found to be more effective for use with multi-dimensional data than many alternatives. An R-tree may index data having almost any number of dimensions but in order to provide the most efficient response to queries on the indexed data, it may be desirable to limit the dimensionality of the data to the range of two to approximately ten.

Previous methods of storing R-trees often used file-based systems in which an R-tree was stored among several files, external to a database environment. Because the tree is not part of a database in such methods, such solutions preclude the ability to apply standard tools and utilities for database update, retrieval, security, concurrent access, etc. Some attempts have been made to store R-trees within the framework of a database system, but typically did so by storing an R-tree as a binary large object (BLOB) or an external file. With a BLOB, one R-tree node cannot be distinguished from another when accessing or manipulating the R-tree.

Another method of indexing multi-dimensional data involved normalizing the data to a single dimension and then indexing it with a B-tree structure. This solution is inadequate because of the resultant loss of spatial proximity, which can adversely affect the response time for queries.

An R-tree suitable for storage in a database management system in a present embodiment of the invention consists of a root node and any number of leaf nodes, which may be connected to the root through a suitable quantity of intermediate nodes. The number of child nodes that the root or an intermediate node may have, and the number of data items a leaf node may have, is determined by the R-tree's fanout, which is represented herein as M. M therefore denotes the capacity of an R-tree node. M may be any value greater than or equal to two, but for two-dimensional data a suitable value is approximately twenty-five. In general, values for M in the approximate range of ten to fifty work well with various embodiments of the invention.

Each data item indexed in this R-tree, and each region or collection of data corresponding to a subtree within the R-tree, is associated with a minimum abounding area, or MBA, that is defined in d dimensions (where d is the dimensionality of the indexed data) and that encompasses the corresponding data. Entries within a node (for either a child node or a data item) may be of the form <child, MBA>, where child is a pointer to or other identifier of a child node (for an intermediate or root node) or a data item (for a leaf node).

Illustratively, the MBA portion of an entry comprises a definition of a boundary, the scope of which depends upon the type of entity of the associated child. If the child is a node, the MBA encompasses all of the data items within the subtree rooted at the child node (i.e., data items depending from the child node and/or all descendants of the child node). If the child is a data item, then the MBA simply encompasses that data item. The different areas or regions of a dataset represented by different MBAs may overlap.

In an R-tree that stores two-dimensional geographic data or the multi-attribute sale data described above, for example, an MBA may consist of a specification or definition of a two-dimensional shape (e.g., a rectangle) enclosing the points, areas or data associated with a child subtree. The shape may be specified as a set of geographic locations or points which, when connected, define a bounding area. An MBA may alternatively be expressed as a series of intervals or segments, one for each dimension in which the data has an indexed attribute or value. In general, therefore, an MBA may be expressed as a series of arrays or vectors, each array having d numbers or values (where d is the dimensionality of the data).

Depending upon the database management system used to store an R-tree, special data types may be defined for the indexed data. For example, one version of the Oracle® Server database includes one or more data types designed to support geoimages and/or geometric data.

In one embodiment of the invention, each node of an R-tree index includes the following fields, where applicable, prior to storage in a database:

Level Height of the node (e.g., leaf nodes are at level one, their parents are at level two, etc.) Node_id Unique node identifier Child_count Number of child entries in the node Sibling Identifies (e.g., via a pointer) the node's right-hand sibling Parent Identifies (e.g., via a pointer) the node's parent (primarily used for root nodes that split because of data insertion) Parent_node_id Node_id of parent node (primarily used for root nodes that split because of data insertion) Entries Array (of up to M two-part entries), where each entry is of the form <child, MBA>

Illustratively, the child element of a root or intermediate node entry in the Entries array is an identifier (e.g., a pointer, Node_id, disk location) of a child node. For a data item entry (i.e., a child of a leaf node), the child element is an identifier of an individual data item.

FIG. 1 depicts two-dimensional dataspace 100 containing data items (a first data item is identified by the reference numeral 102 a) and a representative R-tree that may be used to index the data in one embodiment of the invention. In FIG. 1, four data rectangles are depicted having reference numerals 112, 114, 122 and 124. The data rectangles may represent areas within a CAD/CAM project, areas of a geographical map, portions of geoimages, collections of sales data expressed in two dimensions, etc.

If R-tree 150 is assumed to have a fanout of two, root node 151 cannot store all four data rectangles, thus requiring leaf nodes 161, 162. In the illustrated embodiment, when constructing R-tree 150 for data rectangles 112, 114, 122, 124, the data rectangles are first clustered into MBAs. Because the four data rectangles may be stored in just two leaf nodes, it is sufficient to create two clusters. Thus, minimum bounding area B1 is defined to include rectangles 112, 114, which are stored in leaf node 161, while MBA B2 is defined to include rectangles 122, 124, which are stored in leaf node 162. Root node 151 thus comprises two entries: <node 161, B1> and <node 162, B2>. Note that the MBAs of R-tree nodes may or may not overlap.

Storing an R-Tree Index in a Relational Database

This section introduces a suitable method and form for storing an R-tree index in a relational database management system in one embodiment of the invention. One skilled in the art will appreciate that by storing an R-tree within a database management system, the R-tree may be manipulated using normal database tools and operations. In particular, because the R-tree exists within the database framework, it receives the benefits of database features such as security, concurrency control, backup and recovery, etc.

In this embodiment three database objects are employed to store an R-tree: an INDEX table, a METADATA table and a Node_id generator. In addition, individual data items (e.g., sales data, geographical coordinates, other multi-dimensional data) may be stored in a separate DATA table (e.g., an object-relational table). The composition of INDEX, METADATA or DATA tables discussed herein may be modified (e.g., by adding or removing columns or attributes), merged or divided to suit a particular application or environment without exceeding the scope of the present invention.

Illustratively, an INDEX table stores the nodes of an R-tree index, while a METADATA table stores information about the R-tree itself (e.g., dimensionality, identity of root node, fanout). In this embodiment, each row of the INDEX table corresponds to a separate node of the R-tree. A Node_id generator (which may be created at the time the R-tree index is created) generates unique Node_ids for nodes of the tree. Besides the Node_id, however, a node or an entry in a table may be referenced or accessed by its Row_id (i.e., the location of the entry on a storage device such as a disk drive, memory, etc.), a pointer or other identifier.

In one embodiment of the invention an INDEX table of a database includes one or more of the following columns or attributes, where applicable, for each node of an R-tree index:

Level Height of the node (e.g., leaves are at level one, their parents are at level two, etc.) Node_id Unique node identifier Row_id Location of node in storage device Child_count Number of child entries in the node Sibling Identifier of the node's right-hand sibling Parent_node_id Node_id of parent node (primarily used for root nodes that split because of tree update) Parent_row_id Row_id of parent node (primarily used for root nodes that split because of tree update) Entries Array of two-part entries (up to M in size), each entry being of the form <child, MBA>

An illustrative METADATA table may comprise separate rows for each R-tree index and include the following columns or attributes for each index:

Dimensionality Dimensionality of the index Root Identifier of the root node in an INDEX table Fanout Fanout of the R-tree index

As described previously, nodes and data items may be identified in many different manners—such as by their Row_id, Node_id, a pointer, etc. In particular, the Sibling and Parent columns of an INDEX table and the Root column of the METADATA table may identify nodes by their Row_ids. Also, the child element of an entry in the Entries array of the INDEX table may be a Row_id of a child node or a data item in a DATA table.

In one embodiment of the invention, the size or capacity of leaf nodes and/or other nodes of an R-tree index may be determined by a parameter of the computing environment in which the database is maintained. For example, the capacity may correspond to a page size (or other unit of measure) of a suitable storage device (e.g., disk drive, memory).

FIG. 2 is a flowchart illustrating one method of storing an R-tree index in a relational database system. In FIG. 2, state 200 is a start state.

In state 202, a new index for a set of multi-dimensional data items is received or constructed. The new index may be the only index for the data items or may be one of multiple indexes, possibly constructed according to a different set or order of data dimensions or attributes.

In state 204 the data items are stored in one or more tables in a relational database. The type and configuration of table may depend upon the datatype of the data items.

In state 206 a METADATA table is configured with one or more of the fields described above. This table may already exist (e.g., if another index is already stored in the database) or may be newly generated. In state 208 an entry is stored in the METADATA table for the new index.

In state 210 an INDEX table is constructed to store the nodes of the new index and is configured to include one or more of the fields described above, and possibly other fields as well. In particular, and as described below, in one embodiment of the invention an index node may store data other than that used to construct the index. For example, in an index of multi-attribute sales data each leaf node may store a summary of all sales or profits generated by its individual data item children. Each node above the leaf node level may store aggregations of data items of descendant nodes. Thus, each node stores a summary of sales or profits commensurate with its level in the index. Such summaries or aggregations of non-indexed data may be rapidly reported with having to access individual data items. In state 212 an entry is stored in the INDEX table for each node of the index. The illustrated method then ends with end state 214.

Manipulating an R-Tree Within a Database Environment

This section presents illustrative methods for creating and manipulating an R-tree index within a database system in one embodiment of the invention. One skilled in the art will appreciate that other suitable methods may be applied or may be derived from those described herein without exceeding the scope of the present invention.

Creating an R-Tree Index for Storage in a Database

Various methods of indexing multi-dimensional or multi-attribute data in an R-tree structure are presented in this subsection. Following a discussion of how multi-dimensional data in general may be indexed, a method of indexing multi-attribute data is presented. As explained above, multi-attribute data may be considered a special case of multi-dimensional data. In either case, the resulting R-tree index may be stored in a database using a method discussed in the preceding section. One skilled in the art will appreciate that the methods described in this subsection may be modified and other similar methods developed without exceeding the scope of the invention.

As will be appreciated by those skilled in the art, the present invention may be used to create an index for virtually any type of multi-dimensional or multi-attribute data. In particular, embodiments of the invention are intended for use in indexing non-point data as well as point data.

In one embodiment of the invention a VAMSplit algorithm (extended for polygons by using their centroids) is particularly effective for creating an R-tree index from multi-dimensional data. Other methods of creating an R-tree are disadvantageous for various reasons. A hilbert R-tree algorithm, for example, linearizes the R-tree data, losing spatial proximity in the process, and a tile-recursive algorithm yields poor clustering when the R-tree data is not distributed fairly evenly across the multiple dimensions.

In one effective application of a VAMSplit algorithm, a set of N data points is divided whenever N>M (i.e., there are too many data points to fit in a single node). In order to divide the dataset most equitably, the distribution of data values within each dimension is computed (e.g., the difference between the smallest and greatest values is determined). The data is then sorted in the dimension having the greatest variance and the sorted dataset is then divided in that dimension as close to a median value as possible.

Advantageously, a multi-dimensional data index may allow all desired data to be retrieved in a single query. This is more efficient than invoking multiple queries against uni-dimensional indexes (e.g., and then determining a union of the results).

FIG. 3 illustrates one method of dividing a set of two-dimensional data points to create an R-tree index according to one embodiment of the invention. For the purposes of FIG. 3, it may be assumed that the fanout, M, for the target R-tree is equal to three. The x- and y-axes may represent any suitable indicia, such as latitude and longitude, color and intensity, etc.

In FIG. 3, dataset 300 comprises multiple data points (a first data point is represented by the numeral 302 a) that vary more in the x dimension than the y dimension. They are therefore sorted according to their values in the x dimension and a first division of the data is made at an approximate median, which is illustrated by dividing line 310.

By computing and applying an approximate median value the storage utilization and efficiency for a given R-tree fanout may be maximized. An approximate median, m(N), for N data points may be computed as follows. If the number of data points to be divided is less than twice the fanout value (i.e., N<2*M), an effective approximate median is equal to the floor (i.e., the truncated or rounded down) value of N divided by two (i.e., m(N)=floor(N/2)). Otherwise (i.e., N≧2*M), an approximate median may be determined by the equation

m(N)=M*floor(N/(2*M)+0.5).

In FIG. 2, N=11 and, with M=3:

m(11)=3*floor(11/(2*3)+0.5)=6,

indicating that an effective location for a first division is after the sixth point in the selected (i.e., x) dimension, which is illustrated by dividing line 310.

After the first partition, it is determined whether each new subset of data will fit into a single R-tree node. Each subset that is too large (i.e., each subset that contains more than M data points) is further divided in a similar manner.

Thus, a first data subset (e.g., that which includes values in the x dimension less than m(11)) is further subdivided as follows. This data subset possesses greater variance in the y dimension than the x dimension, and the approximate median (denoted by m(6)) for the subset is computed accordingly. Because N=2*M,

m(6)=3*floor(6/(2*3)+0.5)=3

and the first subset is divided after the third data point in the y dimension. Dividing line 312 illustrates this division.

The second subset (i.e., the subset having values in the x dimension greater than m₁) yields an approximate median value in the x dimension of 2 (represented on the x-axis by m(5)). In particular, N<2*M, therefore

m(5)=floor(5/2)=2

and the second subset is divided after the second data point. This division is represented by dividing line 314. It will be apparent that each of the approximate median values computed above is relative to the subject dataset or data subset. Thus, the representations of m(11), m(6) and m(5) on the x- and y-axes should be considered relative to the origin or the preceding approximate median value, as appropriate.

After the second subset is divided, each subdivision, or cluster, of data points can now fit into a node of an R-tree having a fanout value of three; therefore no further division is required. Clusters 320, 322, 324, 326 in FIG. 3 are demarcated by dotted boundaries. Now that the data is clustered appropriately, the R-tree index may be constructed by placing each cluster of data items into a separate leaf node (e.g., by placing identifiers of each data item and its storage location or address in the appropriate leaf node). From the leaf nodes, parent nodes may be formed such that each entry in a parent node comprises an MBA of a cluster of data and an identifier of (e.g., a pointer to) the corresponding leaf node. In similar fashion, grandparent nodes of the leaf nodes, and higher nodes as necessary, may be formed. Eventually a root node of the R-tree is constructed.

FIG. 4 depicts R-tree 400, one possible result of indexing dataset 300 of FIG. 3. In FIG. 4, leaf nodes 420-426 are created for clusters 320-326. Because the number of leaf nodes (i.e., four) is greater than M (i.e., three), intermediate nodes 412, 414 are created to index the leaf nodes. Root node 402 is then constructed, with entries for nodes 412, 414. R-tree 400 may be stored in a database by a method described above.

The same approach may be extended to cluster non-point datasets consisting of polygons and other spatial data. Illustratively, for such non-point data the centroids of the data items are used to divide the dataset and cluster data items into leaf nodes. However, an MBA of a leaf node may reflect the full range of data items in the node. The remainder of the tree may then be constructed as described above.

As described previously, “multi-dimensional data” can include data that is inherently multi-dimensional (e.g., geographical data) and can also include data having multiple attributes, such as sales data having characteristics including time of sale, region of sale, product sold, etc. In one embodiment of the invention multi-attribute data may be indexed in a manner different from inherently multi-dimensional data. Because of the similar manners in which these types of data may be stored, however, the following discussion may use the terms attribute and dimension interchangeably.

An existing method of indexing multi-attribute data simply consists of constructing an index on one of the attributes. Such an index, however, is inefficient for responding to a query based on a different dimension or a query based on values in two or more dimensions. For instance, a report or a query may be posed on values in both time and region attributes (i.e., a query involves a conjunction of predicates on the time and the region attributes). Having an index on just one of these two attributes does little to facilitate rapid processing of the request because the data that satisfies both predicates may be scattered within the index. Likewise, having separate indexes (e.g., one each for the time and region attributes) may also be quite slow due to the need for multiple index searches and merging of results.

A solution in one embodiment of the invention is to construct a combined index for multiple attributes of the data. In particular, an R-tree index may be generated using one or more dimensions, where each dimension represents a multi-tiered or hierarchically structured data attribute. In other words, a region dimension may comprise attributes or sub-attributes that have a hierarchical arrangement, such as State and City (i.e., every city is part of a state).

In generating such an index, data items may be clustered in a manner that facilitates relatively rapid responses to a query that matches a set of expected query patterns. Illustratively, a query pattern or a set of query patterns specifies a set of dimensions (or hierarchically structured attributes) and an expected selectivity for the query predicates on those dimensions or attribute hierarchies. By way of illustration, in the sales example above queries may always (or always be expected to) retrieve data in terms of years and cities. In this illustration two dimensions/attribute hierarchies are expressed—the first being time, for which the selectivity is yearly, and the second being region (consisting of state and city attributes), for which the selectivity is city.

Thus, if a user is likely to retrieve data based on one or more particular attributes, and/or with a particular range of values within an attribute, the R-tree can be constructed in such a manner as to respond to these queries in a very efficient and responsive manner. The possible data values for each attribute or dimension may be discrete in nature (e.g., specific dates or cities) or may be continuous (e.g., latitude and longitude). In different embodiments of the invention a user may design or specify a query pattern, select one from a number of options, or a default pattern may be assumed.

In one embodiment of the invention multiple R-tree indexes may be created from a given set or subset of multi-attribute data. Thus, if users employ multiple query patterns for the data a different index may be used to respond to different queries. The different indexes may reflect different orderings of the data dimensions, different scopes or hierarchies within a particular attribute, etc. A query analyzer or similar tool may be employed to determine which of multiple established query patterns a user's actual query most closely matches. In addition, one or more query patterns may be “learned” from a user's activity and a suitable index may then be constructed.

In storing multi-attribute data in a database, each dimension or attribute hierarchy may be stored in any of a number of different forms. For example, each dimension or attribute tier may be stored as a separate object, as a separate row in a table, its values may be stored as separate columns in a table (e.g., one column for each year, each quarter of a year, each month), etc. The range of values of a dimension may also be stored as a hierarchy, such as year, quarter, month, and day for a time dimension.

One algorithm for generating an R-tree index to store multi-attribute data in a database is provided below. In this algorithm data is separated or clustered on the basis of query retrieval units. Illustratively, a query retrieval unit represents the minimal scope or granularity (i.e., selectivity) with which data is indexed in a particular dimension, which scope is specified in the operative query pattern. For example, if the specified query pattern retrieves data for three different products (i.e., the product dimension has three values), then there are considered to be three query retrieval units in the product dimension. As another example, consider a time dimension. If the sales data reflects sales over a two-year period, we could define our query retrieval units to be years, quarters, months, etc., depending on the specified query pattern. If we choose quarters, then there would be eight query retrieval units; if we choose months, then there are twenty-four.

An illustrative algorithm for clustering multi-attribute data items for leaf nodes of an R-tree index is now provided:

1. Store all data items in a single node if possible (i.e., depending upon the fanout of the index)

2. Otherwise, until each subset or cluster of data items will fit into a single node, do:

2.1 Select a dimension in which to divide an over-populated cluster or subset of data items:

2.1.1 Compute the number of query retrieval units in each dimension:

2.1.1.1 For attributes having discrete values, determine the number of possible values (e.g., number of cities for a region dimension or brand names for a product dimension)

2.1.1.2 For attributes having a continuous range of values (e.g., latitude), a query pattern may specify a percentage, P, of the values in that domain that may be accessed in a particular query; the number of query retrieval units is then equal to (100/P)*(range of subset/range of entire set)

2.1.1.3 Some attributes (e.g., time) may be represented by discrete values (e.g., days, months, quarters) or a continuous range; the number of query retrieval units is measured accordingly

2.1.2 The attribute that has the most query retrieval units is selected as the dimension in which to divide the data items

2.2 Sort the data items in the selected dimension

2.3 Divide the data items, possibly to yield an (approximately) equal number of data items in each subset or, as one alternative, in a manner that yields a number of data subsets one or more of which will fit into individual leaf nodes

3. Repeat steps 1 and 2 until each subset fits into a single node.

It should be noted that for attributes having values that fall within a continuous range (e.g., latitude, longitude, possibly time) instead of being expressed as discrete values, a query pattern selected by a user or adopted for the construction of an index may specify a likely percentage of the range of attribute values. For example, if values for a latitude dimension have a total range of ten degrees, a query pattern may specify that a query (e.g., a query window) is expected to be only one-half of one degree wide. Thus, five percent of the dimension's range may be invoked during each query and the resulting number of query retrieval units in this example is twenty.

At the time that data items are to be divided in a selected dimension, different criteria may apply for selecting an optimal point of division. In general, when the dimension in which the data items are divided is marked by discrete values, the point at which the data items are divided (e.g., in Step 2.3 above) is selected to ensure a clear demarcation between items in one subset and those in another subset. When the dividing dimension is marked by continuous values, however, the selected dividing point is intended to minimize the overlap of the MBAs in the resulting index. In one embodiment of the invention data items are divided by calculating an approximate median as described earlier in this subsection. The calculated approximate median may be adjusted somewhat in order to achieve a more efficient ordering of the data items. In addition, when query retrieval units comprise intervals or ranges rather than discrete values, the center of the intervals may be used to calculate variance and for other purposes.

After leaf nodes of the index are constructed for the data items, nodes at the next higher level of the index may be generated based on the leaf nodes. In like fashion, successively higher levels of the index may be populated until a root node is put in place.

Multiple indexes can be constructed by choosing different selectivity values for a dimension or attribute hierarchy. For example, one index may be constructed to cluster on year and city granularities/selectivities (e.g., for time and region dimensions) as in an earlier example. Another index may be constructed using year and state granularities. The number of possible index structures increases as the hierarchy (e.g., number of tiers) in each dimension or attribute increases. Thus, if a county attribute is added to the region dimension then query patterns could be designed accordingly, which may affect the number of query retrieval units and the manner in which the dataset is divided. As one skilled in the art will appreciate, multiple indexes are useful in parallel evaluation of expensive OLAP (OnLine Analytical Processing) operations, such as CUBE in warehousing applications.

In one embodiment of the invention an index of multi-dimensional or multi-attribute data may also store information derived from data items, in addition to storing information concerning the data dimensions or attributes. For example, nodes of an R-tree index for sales data may store sales or profit data in addition to values for searchable attributes such as state, city, year, product, etc. Searchable attributes refer to those attributes or dimensions that may be specified as part of a search or query—such as in a WHERE clause of a SELECT statement in SQL (Structured Query Language).

The sales or profit data stored in index nodes in this example may be aggregated. In other words, a leaf node may store the combined sales or profit figures for all of its data items. The next node above that leaf node may store the aggregated sales or profit figures for all of its leaf node children, and so on. In this embodiment, only the searchable attributes (e.g., state, city, year, product) are used to form the index (i.e. to divide and cluster the data items, form MBAs). Storing the secondary data (e.g., sales, profits) in index nodes allows the rapid generation of summary reports without having to access a DATA table or individual data items.

FIG. 5 is a flowchart illustrating one method of constructing a hierarchical index for a set of multi-dimensional data. In FIG. 5, state 500 is a start state.

In state 502, a set of multi-dimensional or multi-attribute data items is selected for indexing. In state 504 the number of data items (e.g., point data items, polygons, etc.) in the dataspace is counted or otherwise determined.

If, in state 506, it is determined that all of the data items will fit into one node (i.e., the total number of data items is no greater than the node capacity of the index), the data items are placed (e.g., associated with) one leaf node in state 522, after which the illustrated procedure continues at state 524.

Otherwise, if the number of data items in the dataspace is greater than the node capacity of the index, in state 508 the variance within each dimension or attribute (or attribute hierarchy) is determined.

In state 510 the dimension or attribute hierarchy having the greatest variance is selected and, in state 512, the data items are sorted in the selected dimension.

In state 514 the data items are divided in the selected dimension into two or more subsets. An approximate median may be computed as described above in order to divide the data items in half as nearly as possible. Alternatively, the data items may be divided into a number of clusters, each of which contains a number of data items that will fit into one leaf node.

In state 516 the number of data items in each subset is calculated to determine if any of the subset need to be further subdivided to yield clusters that will fit into leaf nodes.

If, in state 518, it is determined that none of the subsets have a greater number of data items than the node capacity of the index, the data items in each cluster/subset are placed in a separate leaf node and the illustrated procedure proceeds to state 524.

Otherwise, if a subset has too many data items, one such subset is selected in state 520. States 508-520 are then performed repeatedly, as necessary. In this manner, the set of all data items is divided and subdivided as necessary to yield clusters of data items small enough to fit into individual leaf nodes.

In state 522, each data item is stored or associated with a leaf node. Illustratively, an entry is made in the leaf node, to contain an identifier of the data item (e.g., by Row_id) and a suitable MBA, which will closely fit the data item in one embodiment of the invention.

In state 524, the higher levels of nodes in the index (e.g., levels 2 and up) are configured. Illustratively, a node is configured at level 2 for every L leaf nodes, where L≦M. In particular, nodes at level 2 and above may be formed so as to minimize the total area or volume associated with the node. Each newly configured node receives an entry for each child node, consisting of an identifier (e.g., Node_id, Row_id) and an MBA that encompasses the MBAs of each entry in the child node. State 524 finishes with the configuration of a root node. The illustrated procedure then ends at end state 526.

Inserting or Deleting a Data Item

Update operations (e.g., insertion or deletion of data) traverse an R-tree as necessary to carry out the addition or removal of a data item and ensure that the R-tree index is updated as necessary to reflect the modification. This subsection presents illustrative methods of inserting and deleting a data item in an R-tree index that is or is to be stored in a database. The data item may be any type of multi-dimensional (including multi-attribute) data.

When adding a data item in a present embodiment of the invention, an R-tree index is traversed from the root to find an appropriate leaf node in which to insert the data item. Illustratively, the choice of which branch or subtree to follow from a given node is made based upon which child node would require the smallest increase in its MBA if the new data item were added. However, if the node at the root of a selected subtree has as children leaf nodes whose MBAs overlap, then the child that is chosen to receive the data item is the one whose expanded MBA (i.e., to receive the new data item) results in the least overlap (if any). If, perchance, the new data item could be added to more than one leaf node and result in the same or nearly the same amount of overlap, then the leaf node whose MBA increases in size the least is chosen.

When a leaf node chosen to receive a new data item would exceed its capacity (i.e., M, the R-tree fanout), the node is split into two nodes by dividing the entries (i.e., the existing entries plus the new item). In one embodiment, the data items are divided in such a manner that the overlap between the nodes' MBAs and the size of their respective MBAs are minimized. A method similar to that described above for the initial partitioning of R-tree data may be applied to divide the nodes.

When a leaf node splits, ancestors of the affected node may need to be updated as well (e.g., to adjust MBAs). Thus, the split propagates upward until the old and new leaf nodes are correctly connected to the root and the MBAs of intermediate nodes are adjusted as necessary to reflect the change in the index structure and composition.

A suitable algorithm for inserting a new data item in a leaf node of an R-tree T, as an entry in the form <identifier, MBA> follows (where identifier may be a Node_id, Row_id or other identifier of the data item, and MBA is a suitable bounding area encompassing the data item):

[Step 1—Locate a leaf node for inserting the new data item]

1.1 Set insertion-node to the root of T

1.2 Initialize insertion-path (e.g., a stack) to record the path of this update operation

1.3 While insertion-node is not a leaf node, do:

1.3.1 Compare entries in insertion-node; select the entry that would cause the least increase in its MBA area to add the data item (or the least increase in MBA overlap if insertion-node's children are leaf nodes)

1.3.2 Push insertion-node onto insertion-path

1.3.3 Set insertion-node to the node associated with the entry selected in Step 1.3.1

1.3.4 Push insertion-node onto insertion-path

[Step 2—Insert data item in leaf node and update nodes on insertion-path]

2.1 Initialize insertion-entry to the new data item

2.2 Initialize modified-entry to nil (i.e., empty)

2.3 While insertion-path is not empty, do:

2.3.1 Pop the top node in insertion-path, call it nodes

2.3.2 Set old-MBA to the MBA of nodes

2.3.3 If modified-entry is not empty, then:

2.3.3.1 Identify the entry in node_(i) whose child reference (e.g., pointer, Row_id, Node_id) matches that of modified-entry

2.3.3.2 Replace the MBA of that entry in node_(i) with the MBA of modified-entry

2.3.4 If insertion-entry is not nil, then (insert it in node_(i)):

2.3.4.1 If nodes is not full (e.g., has less than M entries):

2.3.4.1.1 Add insertion-entry to node_(i)

2.3.4.1.2 Increment child counter of node_(i)

2.3.4.1.3 Set insertion-entry to nil (i.e., empty)

2.3.4.2 If node_(i) is full:

 2.3.4.2.1 Create new node, call it node_(ii)

 2.3.4.2.2 Divide the entries in node_(i), plus insertion-entry, into two subsets; store one in node_(i) and the other in node_(ii)

 2.3.4.2.3 Set insertion-entry to the entry <node_(ii), MBA of node_(ii>)

2.3.5 Set new-MBA to the new (e.g., recalculated MBA) of node_(i)

2.3.6 Compare old-MBA and new-MBA to determine if the MBA of nodes changed during the update. If so, set modified-entry to <node_(i), new MBA of nodes_(i)>; otherwise, set it to nil (i.e., empty)

2.3.7 If modified-entry and insertion-entry are nil, end the algorithm (i.e., the update does not propagate to the parent of insertion-entry)

2.4 If insertion-entry and modified-entry are not nil (i.e., the root node of T has split), create a new root node with entries for these two entries and update the METADATA table accordingly.

Although this algorithm is configured to add a new data item to an R-tree index, one skilled in the art will appreciate how it may be modified to produce a suitable algorithm for deleting a data item.

FIG. 6A depicts the dataset of FIG. 3, with new data item 602 that is to be added to R-tree 400 by applying the algorithm provided above. FIG. 6B depicts R-tree 650 resulting from the data insertion operation. The process of adding data item 602 may be described as follows.

The location phase of the algorithm commences at the root. At root node 402, the entry for node 414 is chosen for insertion because the MBA of the subtree rooted at node 414 need not be increased to include data item 602. At node 414, it is determined that the MBA for leaf node 424 would require less expansion than the MBA of leaf node 426. Thus, at the end of the location phase, the insertion path consists of node 424, node 414, and root node 402.

It should be apparent that adding an entry for data item 602 to leaf node 424 in the insertion phase of the algorithm will cause node 424 to split because it would contain more than M (i.e., three) entries. Thus, node 428 is created to accept the extra data. The data items of node 424 plus item 602 are then divided among nodes 424, 428. In particular, if the partitioning method described above is applied the data items are split along the x-axis. Cluster 324 is thus replaced by clusters 324 a, 324 b.

In the second iteration of the insertion phase of the algorithm, node 414 is visited. The MBA of the entry in node 414 for leaf node 424 is modified to reflect the change in data item distribution. Also, an entry for node 428 is added to node 414. And, because the MBA of the entire subtree rooted at node 414 did not change, the algorithm ends. The end result, shown in a graphical format, is depicted in FIG. 6B.

The data items in the individual leaf nodes are not depicted in either FIG. 4 or FIG. 6B. As described above, in one embodiment of the invention data items are maintained in a separate DATA table, in which case each item is identified in a leaf node by its Row_id or some other suitable identifier. A primary difference in the leaf nodes between FIG. 4 and FIG. 6B is that leaf node 424 has been modified to only include the data items in cluster 324 a (vice cluster 324) and leaf node 428 has been added for cluster 324 b.

The processing of a deletion operation proceeds in a manner analogous to the insertion of a new data item. In particular, a deletion operation commences at the root node and propagates downward through all nodes and subtrees that intersect an MBA containing the data item to be removed. Eventually, the leaf node containing the target data item is identified and the data item is deleted.

In one embodiment of the invention a leaf node is not deleted even when the last data item within the node is removed. The MBAs of all ancestor nodes are, however, updated as necessary. In this embodiment a reorganization function is provided to restructure an R-tree for more efficient operation. Illustratively, the reorganization function identifies empty or under-filled nodes (e.g., nodes with less than 40% occupancy) and/or nodes with inefficient structure. A node may be considered to possess an inefficient structure if, for example, the area of its MBA can be substantially decreased by deleting a relatively small number of entries. If the quantity or ratio of empty and inefficient nodes meets a certain criteria (e.g., a range of approximately 5% to approximately 10% of the total number of index nodes) then the tree is restructured. In an illustrative restructuring operation empty nodes may be deleted and entries may be deleted from under-filled or inefficient nodes and re-inserted into the index. In one implementation of this embodiment other update operations on the R-tree are suspended during the reorganization function. Giving only the reorganization function the ability to update the R-tree helps prevent the index from being corrupted during the operation.

In a previous method of updating a particular type of R-tree known as an R*-tree, a data item could be forcibly inserted into a node (e.g., if the node is already full). And, when a data item was deleted from an R*-tree, its node may be deleted (if empty) or merged with another. As discussed above, however, a method of updating an R-tree in one embodiment of the present invention splits nodes instead of forcibly inserting entries and postpones treatment of empty or under-filled nodes. In addition, the previous method of updating an R-tree was designed to work with a file-based index, not an index that has been stored in a database system.

An R-tree suitable for storage in a database management system in one embodiment of the invention may support a number of other operations, such as a window query, a nearest-neighbor query or an intersection join. These operations are discussed in the following subsections.

In one embodiment of the invention, query operations return or identify data items whose MBAs satisfy the specified query parameter(s). The degree of correlation between the query parameter(s) and the data that is returned may depend upon the type of data involved. For example, for most data that is not inherently multi-dimensional (e.g., from data warehousing applications, multi-media applications, etc.), the data identified in response to a query will likely be quite accurate even though the data items were located on the basis of their MBAs rather than their specific values. For applications such as GIS, however, the use of MBAs may limit the precision with which identified data items match a user's query. For these applications some post-query processing may be applied to determine the most appropriate results for a user in a given application or domain.

Window Query

One type of query that is supported in one embodiment of the invention is a window query. Illustratively, in a window query a user specifies a window (e.g., a set of ranges or points in each dimension, a rectangle drawn on a graphical interface) and data having a specified relationship to the window is retrieved or identified. The specified relationship is provided as a selection criterion of the user. Illustrative selection criteria include intersection, containment, enclosure and match. A window may be of virtually any size or shape.

Illustratively, a window query submitted with a criterion of “intersection” (i.e., an “intersection query”) retrieves data items having MBAs that intersect the specified query window. Similarly, containment queries identify data items whose MBAs are completely contained within the query window. Conversely, enclosure queries retrieve data items whose MBAs completely enclose the query window. Finally, match queries identify data items whose MBAs match the query window (e.g., exactly or within a specifiable range of similarity).

For a given query window and set of data items or datasets, in one embodiment of the invention any data item that satisfies a match query will also satisfy an intersection, containment and enclosure query. Further, any data item that satisfies an enclosure or containment query will also satisfy an intersection query.

To answer or respond to a window query in one embodiment of the invention a search for target data items commences at the root and propagates toward the leaf nodes in a recursive manner. Starting with the root, at each non-leaf node visited in response to the query it is determined which child nodes should be examined, depending upon the specified criterion. In a parallel processing environment multiple child nodes may be considered at once.; otherwise, only one is investigated at a time and other child nodes that should be visited are marked for later examination. A stack, list or other data structure may be maintained during the processing of a query to store nodes that are to be examined.

Illustratively, if the selection criterion is either intersection or containment, query processing propagates to those child nodes/subtrees whose MBAs intersect the query window. If the selection criterion is either enclosure or match, the query propagates to those child nodes/subtrees whose MBAs enclose the query window.

At the leaf level, each data item in a visited leaf node is examined to determine if it satisfies the applicable selection criterion. Thus, while similar tests may be applied at non-leaf nodes for intersection and containment criteria and for enclosure and match criteria, at leaf nodes the query processing may differ for each criterion.

Nearest-Neighbor Query

Another type of query that may be implemented in an embodiment of the invention is a nearest-neighbor query. The purpose of this type of query is to locate the nearest data item(s) to a specified point or data value. Based on the target value or point specified by a user and the number of neighbors requested, an R-tree is searched and the appropriate data items returned to the user. For example, in a geographical application a user may specify a place of interest in order to find a desired number of the closest neighboring points.

One method of responding to a nearest-neighbor query is as follows. A priority queue (which in an alternative embodiment may be a list, stack, or other data structure) for tracking subtrees and/or data items from the R-tree is maintained; the queue is initially empty. Starting with the root node, child nodes and data items are inserted and removed from the queue as the query propagates through the index. A set of k candidate neighbors is also maintained (e.g., as array entries ordered by their distance from the query), where the value of k may be supplied by the user or may be a default value.

First, an entry is added to the queue for each child of the root node. Illustratively, entries in the queue are of the form <item, distance>, where item may be a node or a data item, depending on the height of the current node within the tree. Where, the item is a node, distance is the distance from the user's point to the closest edge or perimeter of the node's MBA. Where, however, the item is a data item, distance is the distance from the user's specified point to the data item or the closest boundary of an MBA of the data item.

The entry in the queue having the shortest distance parameter is selected for further processing. Thus, a priority queue in which entries are ordered on the basis of the distance from the entry to the query may be best suited for this purpose. If the selected entry is another node (e.g., a child node of the root), the entry is removed from the queue and replaced by entries corresponding to the child entries of that node (which may be child nodes or data items). If the selected entry is a data item, however, the data item is included in the set of candidate neighbors as long as the set contains less than k neighbors. Otherwise, if the distance from the query to the data item is less than the distance from the query to any other data items in the set, the data item replaces the candidate neighbor farthest from the query.

This process is repeated by removing entries from the queue until the farthest neighbor in the set of candidate neighbors is closer to the query than any entries in the queue. At that time, the set of candidate neighbors is returned as the set of k-nearest neighbors to the query.

As one skilled in the art will appreciate, the presently described method of finding the k nearest-neighbors accesses a minimum number of nodes for a given index. A previous algorithm, which was developed in the context of quadtrees but is also applicable to R-trees, requires space on the order of O(n) for the priority queue, where n is the number of multi-dimensional data items. Advantageously, the present technique requires space for the priority queue on the order of O(number of nodes) and space for the candidate neighbors on the order of O(k). Thus, because the number of nodes in an R-tree index in an embodiment of the invention can be computed by dividing the number of data items by the index's node capacity, M, the total space required is O(k+n/M). As a result, this technique for finding a set of nearest-neighbors can be performed with limited memory resources and is highly scalable.

This method of finding a nearest neighbor may be illustrated with reference to FIG. 7 and FIG. 4. In FIG. 7, dataset 300 of FIG. 3 is used for purposes of illustration. The R-tree of FIG. 4 is one indexed representation of this dataset.

The query point for the illustrated nearest neighbor query in FIG. 7 is point 702. The MBAs of the subtrees rooted at node 412 and node 414 are depicted by boundaries 712, 714, respectively. Similarly, boundaries 720-726 represent the MBAs of leaf nodes 420-426. For the sake of brevity, it will be assumed that only the closest data item to the query point is desired (i.e., k=1).

Application of the subject method for finding a nearest neighbor begins at root node 402. Accordingly, entries for the two child nodes of the root are placed on the working queue: <412, distance(702, 412)> and <414, distance(702, 414)>.

Because MBA 712 of node 412, is closer to query point 702 than MBA 714 of node 414, the entry <412, distance(702, 412)> is removed from the queue. In its place are inserted entries for the leaf node children of node 412: <420, distance(702, 420)> and <422, distance(702, 422)>.

The queue entries are again examined and it is determined that query point 702 is closer to MBA 714 than MBA 720 or MBA 722. Thus, the entry <414, distance(702, 414)> is removed from the queue and replaced with entries for its leaf node children: <424, distance(702, 424)> and <426, distance(702,426)>.

Of all the queue entries, the closest MBA to the query point is now that of leaf node 424. Thus, the entry <424, distance(702, 424)> is removed from the queue and replaced with entries for each of its data items, including data item 704.

Because the distance between data item 704 and query point 702 is the shortest distance in the queue, it is returned as the nearest neighbor for the query point.

Another method of satisfying a nearest-neighbor query processes the R-tree in a depth-first manner, through the child subtree that is closest to the user's point of interest. In this process a queue or list may be maintained to track the subtrees that are circumvented as a path is traversed, which subtrees may be visited after reaching the bottom of the path.

Yet another method of responding to a nearest-neighbor query combines the previous two approaches. In particular, this approach begins at the root in a depth-first propagation pattern as in the latter of the two previous methods. Processing continues in a depth-first manner until a subtree is reached that will fit entirely within available memory (e.g., cache, RAM). That subtree is then processed using the former method. Thus, this method serves to identify a level within the R-tree at which subtrees may be processed in memory. Above that level (i.e., closer to the root) nodes are ordered first by level then by distance to the user's query point. Below the critical level, nodes are ordered using their distances to the query point.

Intersection Join

Yet another operation that may be performed with R-trees in an embodiment of the invention is a join operation. In a typical join operation, datasets indexed in separate R-trees are compared to identify areas of the datasets that overlap.

In one method of performing a join operation on datasets R and S, processing begins at the root nodes of the corresponding R-tree indexes. A pair of nodes, one from each index, is processed at a time. In processing a pair of nodes, intersecting pairs of entries of the two nodes (i.e., where the MBA of one entry from one node intersects the MBA of an entry from the other node) are identified. If the entries correspond to data items the entry pair is included as part of the join result. If only one of them is a data item, it is used as a window query (with a selection criterion of intersection) on the subtree corresponding to the other entry. A queue (or stack, list, etc.) may be maintained to store entry pairs as they are identified and until they are further processed.

Concurrency Control

In one embodiment of the invention, in which concurrent update operations may be performed on the database in which an R-tree index is stored, one or more measures may be taken to prevent corruption of the index.

For example, some operations—such as the initial construction/storage and/or reorganization of the tree—may be performed in an exclusive mode. In an exclusive mode of operation, no other update operations may be performed, thus eliminating the risk of corrupting the index.

Another measure that has already been introduced is postponing the deletion of empty nodes until an index is reorganized. This measure in large part limits the danger of distorting the index to insertion operations when a node is split. Therefore, in one embodiment of the invention the following additional safeguards are taken.

First, whenever a node, call it node 1, splits into two nodes, call them node 1 and node 1′ the Node_id of node 1 before the split is assigned to node 1′. Node 1 then receives a new Node_id generated by a Node_id generator. Second, when the tree is being traversed during an update operation (e.g., addition or deletion of a data item), the node Node_id is also saved for each node that is added to the update path (e.g., insertion-path from the insertion algorithm provided above). Third, when a root node such as nodeR splits into nodeR and nodeR′, a new root, nodeS, is created with entries for nodeR and nodeR′. The leftmost node of nodeR and nodeR′ sets its Parent_row_id field to the Row_id of nodeS and its Parent_node_id to the Node_id of nodeS.

As a result of these safeguards, if a node splits during an update operation the split is detected and the integrity of the index is maintained. For example, in the insertion algorithm described above, each time a node is added to the insertion-path in Step 1, its Node_id value is also saved. Later, when nodes in the insertion-path are updated in Step 2 to account for MBA alterations and/or split propagations, it will be seen that a node that split will now have a different Node_id than was stored during Step 1. By tracing the Sibling field (e.g., a pointer) of the altered node, recursively if necessary, the new sibling node can be found and updated as necessary (e.g., to store the correct MBAs for its entries). In a like manner, the Parent_row_id field allows the detection of root node splits.

One embodiment of the invention may be configured for use within the extensible indexing framework of Oracle Server. Within this framework, read operations that are conducted during an index query cannot read inconsistent data because all such reads are performed on the same committed snapshot of the database. Within this extensible framework, however, index updates may read the latest committed versions of index nodes. In order to resolve any inconsistencies that may occur during the updates, a technique such as that described above may be used to detect node splits. A split of a root node, however, may be detected by tracing a pointer to a parent node (e.g., the Parent_node_id field of a node) from the erstwhile root node. Thus, a root node is not “anchored” once it is created.

Buffering Hierarchical Nodes For Index Operations

In one embodiment of the invention nodes of a hierarchical index (e.g., an R-tree) are buffered during manipulation of the index (e.g., addition or deletion of a data item, index reorganization, query processing). In particular, if the index is too large to fit into available memory space, one or more nodes of the index may be buffered in accordance with a method described in this section. The index may be stored in a database as described in previous sections.

As one skilled in the art will appreciate, index operations such as query processing typically begin at the root of the index and proceed toward the leaf nodes. The performance of an index operation may be measured by such factors as the number of nodes that are processed or accessed and/or the overall time needed to respond to a query or complete an update. By buffering a portion of the index (e.g., one or more nodes) when the entire index cannot fit into memory, the performance of an index operation can be greatly improved. In particular, maintaining index nodes in memory allows them to be accessed much faster than if they remained on disk or some other relatively slow storage device. Thus, the time required to process a query is determined primarily by the amount of time needed to retrieve index nodes from storage (e.g., disk); the time needed to actually process the nodes is relatively minor in comparison. In a hierarchical index such as an R-tree, each node may be of a fixed size—such as a disk page—thus allowing node operations (e.g., reads, writes) to be performed in an efficient manner.

In addition, present database management systems often maintain multiple indexes for a single database, thereby decreasing the amount of buffer space available to buffer a particular index. Selecting an appropriate buffering policy may therefore have a significant impact on the performance of the DBMS.

The buffering techniques described in this section are well suited for use with an R-tree constructed and maintained as described in the previous sections. However, the described techniques are also suitable for a variety of different hierarchical index structures (e.g., hB-trees, bang files), whether stored in a DBMS or not. Further, the techniques are described in the context of query operations performed on data indexed with a hierarchical index, but may be implemented for other types of operations as well. From the following discussion, a person of ordinary skill in the art will appreciate how an embodiment of the invention may be modified to meet a particular requirement or data environment.

The buffering techniques presented below are suitable for implementation in a variety of database environments. One factor that may determine which technique is most effective for a particular environment is the distribution of queries within the dataspace of the hierarchical index. In particular, one technique may be better suited for queries that are relatively uniformly distributed over the dataspace, while another strategy may be better suited for non-uniform distributions.

The new buffering techniques that are provided are intended to avoid the shortcomings of other schemes that have been attempted. One previous technique for buffering a hierarchical index involved pinning (i.e., buffering) nodes at the top of an index. While suitable for small indexes, this method quickly becomes unsatisfactory as the size of the index grows, especially when the index employs anything other than a small fanout value. The unsatisfactory nature of this technique is due in part to the arbitrary manner in which nodes within a level are buffered when the entire level cannot be buffered. As the index becomes larger and larger, leaving more and more un-buffered nodes in a given level, the performance of this technique degrades rapidly.

Another technique that is generally unsuitable, but which, as described below, may be modified to be acceptable for certain environments, is termed the Least Recently Used strategy, or LRU. In this method, the node that is removed from a buffer to make way for a node newly retrieved from storage is the buffered node that was least recently used in an operation (e.g., query). Thus, this technique assumes that the nodes that have been least-recently used in the past are least likely to be used in the future. One deficiency exhibited by this method is its unfortunate tendency to remove a top-level node (even the root node) from a buffer if the buffer space is small compared to the height of the index structure.

A Buffering Scheme for Uniform Query Distributions

In one embodiment of the invention a new buffering technique is provided that is very effective for a database environment in which queries are distributed fairly uniformly across the dataspace. This technique is particularly well suited for use with multi-dimensional/multi-attribute data and R-tree indexes.

In this technique, illustrated in FIG. 8A (described below), nodes are selected for buffering based on their associated areas (e.g., their minimum bounding areas, as described above). In particular, a node having a larger bounding area is buffered before a node having a smaller bounding area. This technique takes advantage of the fact that the probability of accessing a node during a period of uniform query distribution is proportional to the area of the node. This may be termed a “node-area” buffering policy or strategy.

According to this technique, the root node of a hierarchical index should always be cached or buffered first, and remain buffered, because it has the largest associated area (i.e., it encompasses all the data items within the database). Advantageously, index nodes are buffered from the top down and, within a particular level, the nodes having relatively small areas are rejected in favor of those having larger areas. It will thus be seen that a child node will only be cached if its parent is cached. And, if one of them must be removed from cache in favor of a new node due to a lack of space, the child with the smallest node-area will be removed before the parent.

A Buffering Scheme for Non-uniform Query Distributions

When queries are not distributed uniformly within the domain or dataspace of a hierarchical index, the probability of accessing a particular node during a query may no longer be proportional to the area associated with the node (e.g., its MBA). Therefore, some other characteristic or measure must be used to determine which nodes should be buffered.

In one embodiment of the invention, a buffering technique is provided for non-uniform query distributions wherein nodes are buffered on the basis of their frequency of access. This technique thus “learns” how the queries are distributed and applies that knowledge to cache nodes that have been accessed relatively frequently. This technique may be termed the Least Frequently Accessed (LFA) strategy because it serves to replace the least frequently accessed node in a full buffer when a new node must be buffered.

In accordance with this technique, statistics are maintained for use when making caching decisions. In particular, an access count is kept for each index node and is incremented each time the node is accessed during a query. Then, when a node must be removed from cache to make way for a different node, the one chosen for replacement is the one having the lowest access count. In case of a tie (i.e., more than one node has the same low access count), [a] secondary criteria may be examined. In this example, the node having the smallest node-area (e.g., size of the node's MBA) is replaced.

Because the distribution of queries may not be known before database operations begin, and because the uniformity of distribution may change over time, a given database environment may be operated according to different buffering policies at different times. For example, a node-area strategy may be initially applied; then, when the query distribution becomes non-uniform or enough statistics have been collected, an LFA strategy may be implemented.

FIG. 8A depicts an implementation of a buffering strategy described in this section, for a hierarchical index, according to one embodiment of the invention. State 800 is a start state in which the hierarchical index may be buffered according to some other strategy or may not be buffered at all.

In state 802, the database environment is analyzed to determine an appropriate type of buffering. This analysis may be performed automatically or may involve observations made by a database administrator. In the embodiment of the invention depicted in FIG. 8A, a distinction is made between database environments having uniform query patterns and environments having non-uniform patterns. If the pattern is non-uniform, the illustrated method proceeds to state 814.

For a uniform query environment, in state 804 buffering begins on the basis of node size. Thus, the root node is buffered first because it corresponds to all the data indexed within the tree.

In state 806, it is determined whether an additional node will fit into the buffer space. If not, the method advances to state 812, where database operations may begin. Such operations may take advantage of the nodes that have been buffered. However, if another node will fit into the buffer in state 806, then in state 808 the node having the next largest bounding area or region is identified. This node is then added to the buffer in state 810, after which the illustrated procedure returns to state 806.

In the illustrated buffering strategy, a buffer initialization phase ends at state 806 when no more nodes will fit into the buffer space. Then, database operations (e.g., queries) using the buffered nodes may begin. Thus, in state 812, an unbuffered node may be accessed during database queries. If there is insufficient room in the buffer space to store the unbuffered node, the buffered node having the smallest node area is replaced by the new node. The illustrated procedure then ends at end state 826.

For non-uniform query patterns an LFA method of buffering is illustrated in FIG. 8A, starting at state 816. Illustratively, some other type or technique of buffering may be applied, in state 814, until sufficient statistics have been collected to implement the LFA method. For example, a node-area method of buffering may be implemented initially, based on a determination that queries were, or were likely to be, fairly uniformly distributed within the database.

In state 816, a counter is associated with each node of the index. Alternatively, counters may be associated with only a portion of the nodes if, for example, the available buffer space is too small to store more than a fraction of the nodes. In this case nodes at lower levels of the tree (i.e., close to the leaf nodes) may not be included in the buffering scheme.

In state 818, the node counters are incremented as database operations (e.g., queries) are performed. In state 820 it is determined whether a new (i.e., unbuffered) node has been accessed. Until this occurs, the illustrated procedure returns to state 818.

When a new node is accessed, in state 822 the system identifies the buffered node having the lowest node counter and, in state 824, replaces this node with the newly accessed node. The illustrated method may then end at end state 826 or return to state 818 and continue incrementing the node counters during database operations.

A Generic Buffering Scheme

As described above, a node-area buffering technique is well suited for uniform query distributions while an LFA strategy is very effective for non-uniform distributions. However, these schemes may not be as well suited as others for query distributions that change over time. Thus, in one embodiment of the invention a generic buffering scheme is implemented that is very efficient for all patterns of query distribution.

According to this generic scheme, when a node must be removed from buffer space to make room for a newly retrieved node two index nodes are compared to determine which one provides the least benefit by being cached. By successively comparing the loser of a comparison to other cached nodes, eventually the node providing the least caching benefit will be identified and replaced.

Each index node has an associated area (e.g., a minimum bounding area) and one or more statistics concerning its time and/or frequency of access. Illustratively, each time a node is accessed the time of that access is recorded and an access count or other measure of frequency of access is incremented. With these statistics, a first node A may be compared to a second node B in accordance with the following set of equations:

F(A,B)=AreaValue+FreqValue+TimeValue  (1)

AreaValue=AreaWt*((A.Area−B.Area)/B.Area)  (2)

FreqValue=FreqWt*(A.FreqAccess−B.FreqAccess)  (3)

TimeValue=TimeWt*((A.LastAccess−B.LastAccess)/Bufsize)  (4)

If F(A, B) is greater than zero, A is considered to provide a higher caching benefit than B (i.e., it is better to cache A than B). If F(A, B) is less than or equal to zero, then B should remain in cache. Alternatively, if the comparison yields a value of zero, an arbitrary or reasoned decision may be made at to which node provides greater caching benefit. From equations (1)-(4), it is seen that function F has three components—node area, frequency of access and time of last access. AreaWt, FreqWt and TimeWt, represent weights assigned to the different components. In one embodiment of the invention TimeWt+AreaWt+FreqWt=100. In particular, effective results are achieved when AreaWt=30, FreqWt=50 and TimeWt=20.

AreaValue represents the caching benefit of A, in comparison to B, in terms of the difference in their respective areas (e.g., MBAs). The difference between their respective node areas is divided by the area of node B so that the node area component of F(A, B) gives less preference to A if B has a large area than if B has a small area and vice versa.

FreqValue represents the caching benefit of A or B derived from the frequency with which they have been accessed. The more recently A was accessed than B, the more caching benefit is yielded for A.

TimeValue measures the caching benefit attributable to the nodes' recency of access. Thus, the value A.LastAccess represents the last time node A was accessed. Illustratively, a higher LastAccess value indicates a more-recent time of access. This value may represent an absolute time or a time relative to a particular event (e.g., system initialization). Illustratively, BufSize represents the size of the buffer or cache available for buffering index nodes. Thus, TimeValue compares the difference between the two nodes' latest access times, with respect to the size of the buffer space. The greater the size of the buffer that is available for caching index nodes, the less impact this component has.

The mechanisms for counting the frequency of access to nodes, and/or the mechanisms used to record the time of their last access, may be initialized or re-initialized when the database or DBMS is started, when the index is re-organized or updated, at specified or programmable intervals, etc.

In one embodiment of the invention the loser of a first comparison is then compared to a different cached node. The loser of that comparison is then matched against another cached node, and so on, until each cached node has been compared to at least one other. The loser of the last comparison is then dropped from the cache in favor of the newly retrieved node.

FIG. 8B is a flowchart demonstrating one implementation of this generic scheme to buffer nodes of a hierarchical index according to a present embodiment of the invention. State 850 is a start state.

In state 852 the nodes of the index are buffered according to a strategy described above or some other strategy. In state 854, the system waits until a node not already buffered is accessed (e.g., during a query).

In state 856 two buffered nodes are selected to begin the process of identifying the buffered node that provides the least benefit to the buffering scheme. Any two nodes may be chosen since, in the presently described implementation, all of the nodes will be examined. However, in an alternative embodiment one or more buffering strategies may be combined, in which case nodes may be compared in some particular order.

In state 858 the relative caching benefits of the two nodes are determined according to the calculations described above. The node providing the least caching benefit is selected in state 860.

In state 862 the system determines whether all buffered nodes have been compared to at least one other buffered node. If not, the illustrated procedure continues at state 864, selects a buffered node that hasn't been tested yet, and returns to state 858.

When all of the buffered nodes have been examined relative to another buffered node, the system proceeds to state 866 and replaces the selected node with the newly accessed node. The illustrated procedure then ends with end state 868.

The foregoing descriptions of embodiments of the invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to practitioners skilled in the art. Accordingly, the above disclosure is not intended to limit the invention; the scope of the invention is defined by the appended claims.

Oracle, Oracle Server, and all Oracle Server-based trademarks and logos are trademarks or registered trademarks of Oracle Corporation in the United States and other countries. 

What is claimed is:
 1. A computer-implemented method of buffering a hierarchical index of multi-dimensional data, wherein said index comprises a set of nodes having associated multi-dimensional data regions, the method comprising: storing a set of multi-dimensional data items on a computer system; maintaining a hierarchical index of said multi-dimensional data items; maintaining a buffer configured to store one or more nodes of the set of nodes comprising the index; storing in said buffer a first node in the set of nodes, wherein said first node has the largest associated data region; and repeating said storing for a next node in the set of nodes having a next largest associated data region than the node just stored in said buffer until one of: every node in the set of nodes has been stored in said buffer; and said node having a next largest associated data region is too large to fit in said buffer.
 2. The method of claim 1, in which said maintaining a hierarchical index comprises operating a database, said database comprising: a first object configured to store a record for each node in the set of nodes; and a second object configured to store information concerning said hierarchical index.
 3. The method of claim 2, wherein said operation is a query on said database.
 4. The method of claim 2, wherein a first record in said first object corresponds to said first node of said index and comprises one or more of: a unique identifier of said first node; an identifier of a parent node of said first node; an identifier of a sibling node of said first node; a measure of a number of child items of said first node, wherein each of said child items is either a data item or a child node; and for each of said child items of said first node, a child entry comprising: an identifier of a child item; and a bounding area encompassing said child item if said child item is a data item or a bounding area encompassing all data items descending from said child item if said child item is a child node; wherein said multi-dimensional data region associated with said first node is said bounding area.
 5. The method of claim 4, in which said storing in said buffer comprises copying one or more of said components of said first record into a buffer area in a computer memory.
 6. The method of claim 2, in which said second object comprises one or more of: an identifier of a root node of said hierarchical index; a fanout of said index; and a measure of the dimensionality of said multi-dimensional data items.
 7. The method of claim 1, wherein said buffer is a memory buffer for buffering portions of said hierarchical index stored on a storage device having a slower access speed than said memory buffer.
 8. The method of claim 1, wherein said storing in said buffer comprises: storing in a buffer an identifier of a child item of said first node, wherein said child item is one of a data item and a child node; and storing in said buffer a bounding area for said child item; wherein said bounding area is said associated multi-dimensional data region of said child item if said child item is a child node; and wherein said bounding area is a boundary of a region encompassing said child item if said child item is a data item.
 9. The method of claim 1, further comprising: replacing a second node in said buffer with a third node, wherein said data region associated with said third node is larger than said data region associated with said second node.
 10. The method of claim 1, further comprising: determining a frequency of access of a first subset of said index nodes during a series of operations involving said set of data items; and replacing nodes stored in said buffer with said first subset of said index nodes in order of said frequency of access.
 11. A computer-implemented method of adaptively buffering a hierarchical index of multi-dimensional data, the method comprising: maintaining a hierarchical index of multi-dimensional data items, wherein said index comprises a set of nodes; maintaining a buffer configured to store a subset of said set of nodes; determining a frequency with which each node in said set of nodes is accessed during a first set of index operations; and storing in said buffer a first subset of said set of nodes, said first subset comprising the nodes accessed most frequently during said first set of index operations.
 12. The method of claim 11, further comprising: replacing said first subset of nodes in said buffer with a second subset of said set of nodes comprising the nodes most frequently accessed during a second set of index operations.
 13. The method of claim 12, wherein said first set of index operations comprises said second set of index operations.
 14. The method of claim 11, in which said maintaining a hierarchical index comprises operating a computer database that includes: a collection of records, wherein each of said records corresponds to a node of said hierarchical index; and a description of said index.
 15. The method of claim 14, wherein a first record in said collection of records corresponds to a first node in said first subset of nodes and comprises one or more of: a unique identifier of said first node; an identifier of a parent node of said first node; an identifier of a sibling node of said first node; a measure of a number of child items of said first node, wherein each of said child items is either a data item or a child node; and for each of said child items of said first node, a child entry comprising: an identifier of a child item; and a bounding area encompassing said child item if said child item is a data item or a bounding area encompassing all data items descending from said child item if said child item is a child node.
 16. A method of caching nodes of a hierarchical index during operations performed on a plurality of multi-dimensional data items represented by the index, wherein the index comprises a set of nodes having associated bounding regions, the method comprising: maintaining in a cache a first subset of the set of nodes; receiving a request for an operation involving the plurality of data items; accessing an uncached node not included in said first subset of nodes; determining a difference in size between the bounding region of a first node in said first subset of nodes and the bounding region of a second node in said first subset of nodes; determining which of said first node and said second node provides less caching benefit, wherein greater caching benefit is provided by a node having a larger bounding region than a node having a smaller bounding region; and storing said uncached node in said cache in place of said node providing less caching benefit.
 17. The method of claim 16, wherein said determining a difference in size comprises: subtracting said bounding region of said second node from said bounding region of said first node to produce a size difference; and dividing said size difference by said bounding region of said second node.
 18. The method of claim 16, further comprising: determining a difference in recency of access of said first node and said second node; and determining a difference in frequency of access of said first node and said second node.
 19. The method of claim 18, wherein said determining a difference in recency of access comprises: subtracting a time at which said second node was most recently accessed from a time at which said first node was most recently accessed to produce a time difference; and dividing said time difference by a size of said cache.
 20. The method of claim 18, wherein one of said difference in size, said difference in recency of access and said difference in frequency of access is assigned a weighting factor.
 21. The method of claim 18, wherein said determining which of said first node and said second node provides less caching benefit comprises: aggregating said difference in size, said difference in recency of access and said difference in frequency of access to generate a comparison value; and determining which of said first node and said second node provides less caching benefit by comparing said comparison value to a threshold value.
 22. The method of claim 21, wherein if said comparison value is greater than said threshold value, then said second node is determined to provide less caching benefit than said first node.
 23. The method of claim 22, wherein said threshold value is zero.
 24. A computer readable storage medium storing instructions that, when executed by a computer, cause the computer to perform a method of buffering a hierarchical index of multi-dimensional data, wherein said index comprises a set of nodes having associated multi-dimensional data regions, the method comprising: maintaining a set of multi-dimensional data items on a computer system; maintaining a hierarchical index of said multi-dimensional data items; maintaining a buffer configured to store one or more nodes of said set of nodes; storing in said buffer a first node of said set of nodes having a largest associated data region; repeating said storing for a node in said set of nodes having a next largest associated data region than the node just stored in said buffer until one of: every node in said set of nodes has been stored in said buffer; and said node having a next largest associated data region is too large to fit in said buffer; receiving a request for an operation on said data items; and accessing one or more of said buffered nodes during execution of said operation.
 25. A computer readable storage medium storing instructions that, when executed by a computer, cause the computer to perform a method of caching nodes of a hierarchical index during operations performed on a plurality of multi-dimensional data items represented by the index, wherein the index comprises a set of nodes having associated bounding regions, the method comprising: maintaining in a cache a first subset of the set of nodes; receiving a request for an operation involving the plurality of data items; accessing an uncached node not included in said first subset of nodes; determining a difference in size between the bounding region of a first node in said first subset of nodes and the bounding region of a second node in said first subset of nodes; determining which of said first node and said second node provides less caching benefit, wherein greater caching benefit is provided by a node having a larger bounding region than a node having a smaller bounding region; and storing said uncached node in said cache in place of said node providing less caching benefit.
 26. An apparatus for managing a set of multi-dimensional data items, comprising: a storage device configured to store a set of multi-dimensional data items; a processor configured to manipulate said set of multi-dimensional data items; a hierarchical index of said set of multi-dimensional data items, wherein said index comprises a set of nodes; and a buffer configured to store a subset of said set of nodes to facilitate access to said nodes during an operation on said set of multi-dimensional data items; wherein said subset of nodes is stored in said buffer in accordance with predetermined criteria including one or more of: the size of each node's bounding region, the frequency with which each node is accessed during a first series of operations on the set of data items, and the recency with which each node has been accessed.
 27. The apparatus of claim 26, further comprising a database configured to store said hierarchical index, wherein said database comprises: a first module configured to store one or more of: an identifier of a root node of said index; a fanout of said index; and a dimensionality of said multi-dimensional data items; and a second module comprising a record for each node in said set of nodes, wherein a first record for a first node comprises: an identifier of said first node; and an entry for each child of said first node, wherein a child is one of a data item and a child node and wherein a first entry comprises: an identifier of a first child of said first node; and a bounding region encompassing either said first child if said first child is a data item or a bounding region encompassing all data items of descendants of said first child if said first child is a child node.
 28. A computer readable storage medium storing instructions that, when executed by a computer, cause the computer to perform a method of adaptively buffering a hierarchical index of multi-dimensional data, the method comprising: maintaining a hierarchical index of multi-dimensional data items, wherein said index comprises a set of nodes; maintaining a buffer configured to store a subset of said set of nodes; determining a frequency with which each node in said set of nodes is accessed during a first set of index operations; and storing in said buffer a first subset of said set of nodes, said first subset comprising the nodes accessed most frequently during said first set of index operations. 